1. Field of the Invention
The present invention relates to a method for producing a colored resin particle usable as a latent electrostatic image-developing toner in electrophotography and having a surface to which resin fine particles are attached, with minimum environmental impact.
Further, the present invention relates to a method for producing a colored resin particle usable in electronic paper and having a surface to which resin fine particles are attached, with minimum environmental impact.
2. Description of the Related Art
In electrophotographic image forming apparatuses, colored resin particles containing a colorant are used as toner for forming visible images. In addition, colored resin particles are also used in forming electronic paper images.
Among various types of toner, there is polymerized toner having a small particle diameter and a narrow particle size distribution.
Further, as a method for producing a toner, in which polyester superior in image fixability can be used as a binder resin (as the main component), there is known a method in which at least a binder resin (e.g., polyester) and a colorant are dissolved or dispersed in an organic solvent to prepare an oil phase, the oil phase is added and dispersed in an aqueous phase containing at least a surfactant, and then the organic solvent is removed from the system to obtain resin particle, followed by washing and drying, thereby obtaining a toner (otherwise, referred to as “dissolution suspension method” hereinbelow).
However, a toner containing polyester as a binder resin main component produced by a dissolution suspension method etc. is unlikely to be charged as compared to a toner containing a styrene acrylic resin as the main component. In particular, in a one-component developing system, toner is frictionally charged by agitation and abrasion with a supplying member (e.g. supplying roller) and a developer bearing member (e.g., developing roller), and abrasion with a regulating member (e.g., a developer bearing member and a regulating blade). As compared to a method called a two-component developing system where toner is mixed with a carrier (e.g., iron powder) with stirring so as to be electrically charged, the one-component developing system has less occasion of being charged, and thus the inferior chargeability is a more significant problem that needs addressing.
In addition, when such a toner produced by a dissolution suspension method is used for forming an electronic paper image, it needs to have flowability of particles in addition to the uniformity of particle size distribution of colored resin particles. However, a colored resin particle obtained in an aqueous system by a dissolution suspension method, etc. is poor in flowability and unsuitable for forming electronic paper images.
To solve the problems described above, various studies have been made. As one of the methods, there has been known a method in which a vinyl-based resin superior in chargeability is caused to be present on a surface of toner.
For example, Japanese Patent Application Laid-Open (JP-A) No. 2006-206851 describes a method in which a vinyl-based resin fine particle-containing dispersion liquid is made present in an aqueous phase, and an oil phase is dispersed in the aqueous phase to produce oil droplets, thereby forming a resin layer of the vinyl-based resin on surfaces of toner particles. The vinyl-based resin described therein contains carboxyl groups derived from methacrylic acids in a large amount, and it can also be considered that the vinyl-based resin functions as such a protective colloid that assists the dispersibility of the oil phase droplets. As a result, it can be considered that a resin layer of the vinyl-based resin can be uniformly formed on surface of the oil phase droplets, however, the moisture absorption property of the toner surface is increased due to the large amount of carboxyl groups held by the toner. Thus, the effect of improving the chargeability is not necessarily sufficient, and the toner does not exhibit satisfactory chargeability, in particular, under high-temperature-high humidity environments. An attempt was made to form a shell-structure of the vinyl-based resin, in which the amount of carboxyl groups had been reduced, in the toner production method described in JP-A No. 2006-206851. As a result, the dispersion stability of oil phase droplets degraded, and resulting particles were coarsely formed, and it was impossible to obtain satisfactory particles usable as toner.
Japanese Patent Application Laid-Open (JP-A) No. 2006-285188 describes a method in which an oil phase is dispersed in an aqueous phase to prepare oil droplets, a vinyl-based resin fine particle-dispersion liquid is charged to the resulting oil droplet dispersion liquid before or after a solved is removed therefrom, followed by heating, thereby making the vinyl-based resin present on surfaces of oil droplets or particles from which the solvent has been removed. In this method, the dispersion liquid is heated at high temperature of 70° C. or higher in order to surely make a vinyl-based resin adhere on particles serving as a core, however, in an actual industrial production, the method needs a significant amount of energy, and therefore, it cannot be said that the method is preferable in consideration of production costs and environmental impact.
Further, Japanese Patent Application Laid-Open (JP-A) No. 2008-208354 describes a method of producing core-shell resin particles.
The description is that when an aqueous dispersion liquid is produced and a resin employed is a resin having a basic functional group (e.g., primary amino group, secondary amino group, tertiary amino group, and quaternary ammonium salt group) (generally, it is preferable that the molecular weight per basic functional group be 1,000 or less), the higher the pH of the aqueous medium, the larger the surface coverage is. It is described that in reverse, the lower the pH of the aqueous medium is, the smaller the surface coverage becomes. As described in JP-A No. 2008-208354, the pH of the aqueous medium can also be adjusted by a water-soluble amine compound, however, a toner produced using the water-soluble amine compound is poor in adhesion of resin fine particles to surfaces of core particles, further and has significantly low chargeability. The resultant toner caused problems due to charge defects in electrophotographic processes. In addition, in the production of the core-shell particles described in JP-A No. 2008-208354, resin fine particles used as a shell agent are added during an emulsification treatment, however, when a shell agent is added during an emulsification treatment, the amount of the resin fine particles introduced into cores is increased, and thus it is difficult to uniformly disperse resin fine particles on core surfaces.
Japanese Patent Application Laid-Open (JP-A) No. 05-333587 discloses a toner which is produced by granulating particles composed of a polyester resin and a colorant in a wet process, and subjecting the granulated particles to flocculation, drying, and fusing processes, in which fine particles (charged particles) containing styrene/butyl-acrylate at a compositional ratio (80/20) are fixed to the surface of the toner, in the light of controlling chargeability of toner. However, the toner disclosed in JP-A No. 05-333587 is obtained through granulation of particles in a wet process, granulation, and fusing the dried production by a pulverizer, and the fine particles are not uniformly present on the toner surface. Therefore, it cannot be said that the toner is sufficient in charge stability.
Typically, in a production of a granulated toner, a polyester resin, a polyamine compound and other materials (e.g., colorant, releasing agent, charge controlling agent, and viscosity adjusting agent) are dissolved/dispersed in an organic solvent to prepare an oil phase; and an aqueous phase containing a low-molecular weight activator and a high-molecular weight dispersant such as an organic resin fine particle is prepared. Then, the oil phase and the aqueous phase are mixed and stirred (emulsification step) so that the oil phase is dispersed in the aqueous phase to thereby obtain toner particles while granulating oil phase particles. However, there has been a problem in that when additional resin fine particles are attached onto a surface layer of the granulated toner particles, the adhesion of particles to the surface layer becomes nonuniform due to the polyamine compound contained therein and the resin fine particles are not uniformly attached to the surface of the toner particles.
Further, when a vinyl-based resin is charged to the dispersion mixture from which the solvent has not been removed and when a vinyl-based group, which contains less polar groups such as carboxyl group, the dispersion stability of oil droplets degrades and thus the oil droplets aggregate and coalesce with each other. As a result, it was impossible to obtain satisfactory particles usable as toner.